1. Technical Field
The present invention relates in general to an improved spindle motor design for a computer hard disk drive, and in particular to an improved lubricant retention design for a disk drive fluid dynamic bearing spindle motor.
2. Description of the Prior Art
Generally, a data access and storage system consists of one or more storage devices that store data on magnetic or optical storage media. For example, a magnetic storage device is known as a direct access storage device (DASD) or a hard disk drive (HDD) and includes one or more disks and a disk controller to manage local operations concerning the disks. The hard disks themselves are usually made of aluminum alloy or a mixture of glass and ceramic, and are covered with a magnetic coating.
A typical HDD also utilizes an actuator assembly. The actuator moves magnetic read/write heads to the desired location on the rotating disks so as to write information to or read data from that location. Within most HDDs, the magnetic read/write head is mounted on a slider. A slider generally serves to mechanically support the head and any electrical connections between the head and the rest of the disk drive system. The slider is aerodynamically shaped to glide over moving air in order to maintain a uniform distance from the surface of the rotating disk, thereby preventing the head from undesirably contacting the disk.
The head and arm assembly is linearly or pivotally moved utilizing a magnet/coil structure that is often called a voice coil motor (VCM). The stator of a VCM is mounted to a base plate or casting on which the spindle is also mounted. The base casting with its spindle, actuator VCM, and internal filtration system is then enclosed with a cover and seal assembly to ensure that no contaminants can enter and adversely affect the reliability of the slider flying over the disk. When current is fed to the motor, the VCM develops force or torque that is substantially proportional to the applied current. The arm acceleration is therefore substantially proportional to the magnitude of the current. As the read/write head approaches a desired track, a reverse polarity signal is applied to the actuator, causing the signal to act as a brake, and ideally causing the read/write head to stop directly over the desired track.
Typically, a plurality of the hard disks are stacked vertically on a common spindle that is turned by a disk drive motor at several thousand revolutions per minute (rpm). FIG. 1 depicts a sectional side view of a conventional fluid dynamic bearing design for a spindle motor. One axial end of a shaft 11 (lower end in FIG. 1) is rigidly mounted to the mount flange 12 of the spindle motor. The other axial end of shaft 11 (upper end in FIG. 1) may be provided with optional internal threads so that the disk drive cover (not shown) can be attached to shaft 11 with a screw. The two zones of shaft 11 adjacent to the thrust plates 15 have precise diameters and provide stationary surfaces for the radial fluid dynamic bearings. Shaft 11 is located in a bearing sleeve 13 with the pair of thrust plates 15 therebetween. The flange at the top of sleeve 13 is used to center a disk clamp (not shown). Thrust plates 15 are fixed to shaft 11 and provide the stationary surfaces of the axial fluid dynamic bearing.
Still referring to FIG. 1, a small clearance is located between the outer diameter of thrust plates 15 and sleeve 13. The clearance acts as a reservoir for excess lubricant, and helps to prevent the lubricant from escaping. If any excess lubricant escapes the reservoir, end seals 16 are provided to prevent the lubricant from escaping the spindle motor. Sleeve 13 provides the rotating fluid dynamic bearing surfaces for both the axial and radial bearings. The materials used to form shaft 11, sleeve 13, and thrust plates 15 and their surface treatments must be appropriate to survive wear and ensure reliable function of the disk drive.
As shown on the right sides of FIGS. 2 and 3, the center of the assembly is also provided with at least one lateral vent hole 19 which is interconnected to an axial vent hole 21 that extends through shaft 11. Vent holes 19, 21 are needed to equalize the atmospheric pressure exerted on both sides and both ends of the assembly.
When the assembly is subjected to non-operational vibration, such as during shipping or other handling when the disk drive is not in use, lubricant 17 migrates toward lateral vent hole 19. A barrier film 20 is provided at lateral vent hole 19 on both shaft 11 and sleeve 13 in order to resist the flow of lubricant into lateral vent hole 19. However, during extreme non-operation vibrational loads, such as high amplitude vibration encountered during transportation of the product, barrier film 20 is inadequate to impede the flow of lubricant 17. Under such conditions, some of lubricant 17 can overcome barrier film 20 and enter vent holes 19 and 21, thereby degrading the lubrication performance and causing contamination. Lubricant 17 that migrates into the vent holes 19, 21 will not return to the bearing interface surfaces, which could result in failure of the bearing. Thus, an improved lubricant retention design for a fluid dynamic bearing in a spindle motor which overcomes the limitations of prior art designs is needed.
One embodiment of a lubricant retention design for a fluid dynamic bearing design for a spindle motor is disclosed. A labyrinth gap is formed between the sleeve and the shaft of the bearing and is used in conjunction with a barrier film to impede the flow of lubricant into the lateral and axial vent holes during non-operational vibration, such as shipping and handling of the end product. In another embodiment of the present invention, a plug is located in the lateral vent hole. The plug has a very small passage that permits air to pass therethrough for atmospheric pressure equalization, but prevents the escape of lubricant into the vent holes. Alternatively, the plug may be formed from a non-wettable material such as porous foam or sintered material and provided with a larger opening. Yet another solution utilizes a combination of both the labyrinth and plug designs.